1. Field of the Invention
This invention relates to a cylindrical plug valve and, more specifically, to such a plug valve which includes improved means for sealing the fluid passages therethrough when the plug valve is in a closed position.
2. Description of the Prior Art
There have heretofore been provided a number of cylindrical plug valves which provide resiliently deformable sealing means at each fluid passage therethrough to generally prevent leakage through the valve when the plug is in the closed position. U.S. Pat. Nos. 2,493,966; 2,604,293; 3,103,948; 3,133,723 and 3,168,280 each generally represent one form of this type valve which includes a body having a cylindrical bore and a pair of axially aligned passages intersecting the bore and being normal to its axis. A cylindrical plug valve within the bore can be selectively rotated for alignment and misalignment of a diametral port therethrough with the passages. Sealing means, such as circumferential sealing rings around the plug, above and below the passages, generally prevent fluid from escaping from the interior of the valve. Each of these valves also includes a sealing member disposed within a recess at opposite sides of the plug for alignment with its respective passage when the valve is in the closed position. The sealing member makes contact with the interior of the bore around the passage to generally prevent fluid from entering into the bore from an isolated passage. However, the means by which the sealing member is installed in the side of the plug have been somewhat complicated and difficult to provide. A recess in the side of the plug for receipt of the sealing member in these valves has included a curved surface at its base and, in some cases, curved, separate support devices to insure retention of the sealing member therein. Requiring a curved base for the recess and an additional support member has complicated valve manufacture and added to its cost.
Consequently, there have been sealing configurations, such as those disclosed in U.S. Pat. Nos. 2,505,270 and 2,844,353 in which a recess for a sealing member includes a flat base which can be easily and less expensively machined. These sealing members provide a configuration in which the high fluid pressure in an isolated passage may act on the sealing member to apply pressure thereto in a direction to insure that sealing contact with the interior wall of the bore is maintained. However, there is nothing taught or suggested by the disclosure of these patents regarding effective fluid isolation should one of the sealing members be defective or otherwise fail. Being primarily designed for pressure from the passageway, there is no indication that the other seal, that aligned with the passageway of lower fluid pressure, would prevent leakage from the bore into the passageway of lower fluid pressure if the fluid of higher pressure were allowed to enter the bore. It is of significance, for example, that, by providing a flat base in the recess in the side of a cylindrical plug, the dimensions of the sealing member are larger at the top and bottom thereof because of the greater distance from the base of the recess to the cylindrical bore in this region. The extended lip of the sealing member in these areas would therefore be more susceptible to displacement from pressure from within the bore. Fluid could either leak by a deflected lip or, in a more extreme case, the lip could be depressed toward the passage for a complete loss of sealing contact with the interior surface of the bore.
There is another feature of the sealing member configuration of U.S. Pat. Nos. 2,505,270 and 2,844,353 which could interfere with effective valve operation. Although it is not discussed in these patents, there has been experienced in valves of similar construction a general moving of the sealing member when the valve is being rotated from the opened to the closed position. While the valve is being closed, the bore is generally under a higher fluid pressure and fluid is capable of being entrapped at the bottom of the recess behind the sealing member. As the valve is near the closed position, with the sealing member being about one-half to two-thirds aligned with its respective passage, a large segment of the lip of the sealing member is not supported by the interior wall of the bore. With pressure in the passage being greatly reduced by the closure, the fluid in the bottom of the recess tries to expand and is capable of forcing the leading edge of the sealing member outwardly into the passage to prevent continued closure of the valve.